1. Field of the Invention
The present invention relates to an image display apparatus such as a head-mounted display (to be referred to as an HMD hereinafter).
2. Related Background Art
Conventionally, for HMDs and the like, various kinds of optical systems designed to reduce the overall apparatus size have been proposed. For example, Japanese Laid-Open Patent Application No. 7-333551 (corresponding to EP 0687932A3) discloses an apparatus for displaying an enlarged image of an image on the display surface of an image display means by using a prism-like optical element formed by a so-called free-form surface having no rotating axis of symmetry. Such a prism-like optical element makes it possible to correct image distortion, curvature of field, and astigmatism with a very compact, simple arrangement, and satisfy the telecentric condition required for a liquid crystal display surface when a liquid crystal is to be used as an image display means.
When a transmission type liquid crystal display panel is to be used as an image display means, the panel has drawbacks that the pixel apertures are small due to the presence of driving circuits such as TFTs and the graininess of the pixels is noticeable. For this reason, such a display panel is not suited to displaying moving images on a video player or TV and images with higher resolution on a PC or DVD player.
Under the circumstances, by using a reflection type liquid crystal display panel which can attain a reduction in apparatus size and an increase in pixel aperture, a high-resolution image without graininess can be obtained. For example, an image display apparatus using a currently proposed reflection type liquid crystal display panel is disclosed in Japanese Laid-Open Patent Application No. 11-125791. In this image display apparatus disclosed in this reference, as shown in FIG. 16 in this specification, rays of light from a light source 101 are directly guided to a reflection type liquid crystal display panel 102 without the mediacy of an optical element, thereby illuminating the reflection type liquid crystal display panel 102, and a prism-like decentered optical system 103 allows an observer to visually recognize the reflected image as an enlarged virtual image.
In the image display apparatus disclosed in the above reference, however, since light from the light source 101 is directly guided to the reflection type liquid crystal display panel 102 to illuminate it, the angle defined by the illumination light and the optical axis of the reflection type liquid crystal display panel 102 becomes large, resulting in an increase in the overall size of the display apparatus.
In addition, when a reflection type liquid crystal is used as a display panel, the contrast of a displayed image decreases as the incident angle of illumination light on the liquid crystal increases.
Furthermore, since the reflection type liquid crystal display panel 102 greatly inclines with respect to the decentered optical system 103, the distance from the decentered optical system 103 to the reflection type liquid crystal display panel 102 greatly varies depending on places within the panel surface. This makes it difficult to maintain good optical performance throughout the entire screen.
In another image display apparatus disclosed in Japanese Laid-Open Patent Application No. 11-125791, as shown in FIG. 17 in this specification, a light source 101 is placed on the opposite side of a reflection type liquid crystal display panel 102 to a prism-like decentered optical system 103 to illuminate the reflection type liquid crystal display panel 102 through the decentered optical system 103.
With this arrangement, however, illumination light from the light source 101 is reflected by the respective surfaces of the prism-like decentered optical system 103 to cause stray light and strike the eyes of an observer. As a consequence, ghosts and flare tend to occur.
It is an object of the present invention to provide an image display apparatus using a reflection type display element suited to high-resolution images such as PC outputs, in which the reflection type display element can be illuminated in an ideal state with a very simple, compact arrangement, thereby allowing a user to observe excellent images.
In order to achieve the above object, an image display apparatus according to an embodiment of the present invention comprises a light source for supplying illumination light, a reflection type display element for modulating the illumination light into image light by reflecting the illumination light, an illumination optical system for guiding the illumination light to the reflection type display element, and a projection optical system for guiding the image light to an observer, wherein the image light is incident on the projection optical system via at least a portion of the illumination optical system. In this arrangement, the illumination optical system has a part, in an area where both the illumination light and the image light pass, which changes an optical action depending on a state of incident light. The part which changes its optical action depending on the state of incident light has the function of deflecting the illumination light to decrease an incident angle with respect to the reflection type display element as compared with a case where the part does not exist.
According to another aspect of the present invention, the image display apparatus of one embodiment of the present invention is characterized by having a diffraction optical part in the area, in the illumination optical system, where both illumination light and image light pass.